There has been much interest recently in providing broadband wireless access to fixed networks via cell-based terrestrial communication systems operating at millimeter wave frequencies. FIG. 1 illustrates the basic components of an exemplary communication system 100 referred to herein as a Local Multipoint Distribution Service (LMDS) system. The terrestrial point-to-multipoint LMDS system 100 provides wireless access to fixed networks. The LMDS system 100 includes a number of cells 105-i, i=1, 2 . . . M, each of which include a corresponding hub 110-i also referred to herein as a base station. The cells 105-i are nominally circular in shape. The hubs 110-i generally include hub transmitters which establish point-to-multipoint radio links with subscribers 115 located within a given LMDS cell 105-i. The hubs 110-i are each also connected to a fixed network 120, which may be a computer network, a cable television network or a public telephone network. Point-to-point interconnections or other transmission links could be used in place of fixed network 120 to interconnect cell hubs 110-i. LMDS systems in the U.S. are expected to operate at, for example, frequencies in the 27.5-28.35 and 29.1-29.25 GHz bands.
LMDS system 100 will typically reuse the same channel frequencies in adjacent system cells. This frequency reuse may be implemented using polarization reuse techniques in the hub transmitter in conjunction with either omnidirectional or directional hub antennas. Highly directional subscriber antennas may also be used to minimize interference from adjacent cells. When omnidirectional hub antennas are used, the system could be configured such that orthogonal linear polarizations V and H are transmitted in adjacent cells. When directional hub antennas are used, such as in an arrangement in which four 90.degree. directional antennas each provide communication coverage over a 90.degree. sector of a given cell, the system could be configured such that different orthogonal linear polarizations are utilized for adjacent antenna sectors.
Another technique which reduces interference levels between adjacent LMDS frequency reuse cells is frequency interleaving. Frequency interleaving is described in greater detail in E. N. Barnhart el al., "Frequency Reuse in the Cellular LMDS," Submission to the FCC for inclusion in the LMDS Rulemaking Record, Docket No. 92-297, Jan. 6, 1994, which is incorporated by reference herein. An exemplary frequency interleaving technique involves offsetting the carrier frequencies in different cells by about half of the adjacent channel spacing. For example, in transmission of FM video with channels spaced 20 MHz apart, a suitable frequency interleave offset is 10 MHz. If each FM video signal actually occupies only 18 MHz of its allocated 20 MHz band, the above-cited Barnhart et al. reference indicates that the amount of interference protection obtained from this exemplary frequency interleaving technique is on the order of 10 dB. This interleaving technique is best suited to modulation formats such as analog FM which concentrate signal energy near the center of the channel. These analog formats are likely to be utilized in LMDS systems designed for broadcast video distribution, although digital modulation formats may also be utilized in broadcast video distribution, data distribution as well as other services.
Link budgets for LMDS systems generally assume an unblocked path between the hub 110-i and the subscribers 115. In many locations, this may not always be the case due to blockage from buildings, trees or other obstructions. Low power active repeaters can be used to fill in areas of the cell where there is insufficient signal strength due to excessive blockage along the direct path in the direction of the nearest hub transmitter. A typical repeater would be located inside the boundaries of a cell at a location where the signal can be received from the base station, and would amplify and redirect the signal. The repeater antenna could be cross-polarized to the nearest LMDS hub antenna in order to reduce interference to subscribers not utilizing the repeater.
The LMDS system 100 may be used to provide wireless access to services ranging from one-way video distribution and telephony to fully-interactive switched broadband multimedia applications. Circuit switched applications such as voice telephony, personal video telephony, backhaul for personal communications services (PCS) and ISDN multimedia services could be accommodated. Packet-oriented services such as remote database query, interactive entertainment, personalized information services on virtual channels, transaction processing and electronic data interchange could also be implemented. Additional LMDS applications include primary or emergency backup data transport, two-way distance education and corporate training, and high capacity switched data for image transfers and remote consultation for medical users. Interactive uses include video on demand, home shopping, interactive video games, and residential and business data from sub-T1 to multiple T1 rates. Implementation of digital modulation formats allows the LMDS system provider to take advantage of improvements in digital compression technology and expand to HDTV as these technologies become available.
Presently proposed LMDS system architectures generally differ in terms of cell size, modulation format and hub antenna type. Other system design parameters include antenna patterns, antenna heights, antenna pointing, cell spacing, frequency reuse plan, polarization reuse plan and link budget. The particular configuration of parameters selected for a given LMDS system will generally depend upon which of the previously-mentioned communication applications the system is intended to support, and the underlying architecture philosophy of the equipment and service providers. An exemplary consumer-oriented LMDS system proposed by CellularVision (CV) provides analog FM video distribution with a 4.8 km cell radius, and utilizes polarization reuse, directional subscriber antennas and frequency interleaving to reduce interference between cells. The CV system also utilizes return links (subscriber-to-hub) operating at a lower data rate in the guard bands of the downlink broadcast video channels. A two-way multiple access LMDS system described by Texas Instruments (TI) to the FCC Negotiated Rulemaking Committee (NRMC) on the LMDS/FSS 28 GHz band, July-September 1994, utilizes 52 Mbps QPSK and four directional sector antennas at each hub to provide omnidirectional cell coverage with a nominal cell radius of 5 km. Dedicated spectrum is used for return links, and in asymmetrical traffic applications, users are multiplexed in accordance with a time-domain multiple access (TDMA) technique. Both the CV and TI LMDS systems are designed for operation in the above-noted 27.5-28.35 and 29.1-29.25 GHz LMDS frequency bands. The CV and TI systems are described in greater detail in the Final Report of the LMDS/FSS 28 GHz Band NRMC, Sep. 23, 1994, which is incorporated by reference herein. The 28 GHz band as used herein refers generally to frequency bands which include or are situated in proximity to 28 GHz. The 28 GHz band is thus intended to include frequencies or frequency bands between about 27.5 and 30.0 GHz, such as the above-noted exemplary LMDS bands of 27.5-28.35 and 29.1-29.25 GHz. It should be noted that LMDS systems may also be configured to operate in a variety of other frequency bands.
A significant problem which has arisen in connection with the above-described LMDS systems is the fact that a number of space-based and terrestrial communication systems were proposed for operation within the same portions of the 28 GHz frequency band. For example, portions of this band have been requested by mobile satellite service (MSS) providers for feeder links to satellites providing mobile service, and by fixed satellite service (FSS) providers for fixed-location subscriber uplink transmitters. Microwave equipment manufacturers have also requested allocation of portions of the band for point-to-point microwave service. The above-noted LMDS/FSS 28 GHz band NRMC was formed to study these and other interference issues and to make recommendations to the FCC for allocating and/or sharing the 28 GHz band between LMDS and satellite services. The NRMC investigated a number of interference scenarios covering interference from FSS earth station and MSS feeder link uplinks into LMDS subscriber receivers and interference from LMDS hub transmissions into FSS and MSS satellite receivers. Satellites in both geosynchronous orbit (GSO) and non-geosynchronous orbit (NGSO) were considered. The scenario which showed the greatest potential for harmful intersystem interference was FSS earth stations interfering with LMDS subscriber receivers. LMDS and FSS system proponents envision widespread distribution of LMDS subscribers and FSS earth stations, respectively, throughout the same geographic areas. The description of proposed FSS systems submitted to the LMDS/FSS 28 GHz band NRMC indicates that a single FSS uplink transmitter can cause harmful interference to multiple LMDS receivers.
FIG. 2 illustrates potential interference between an FSS earth station uplink transmitter 140 and LMDS subscriber, hub and repeater receivers in a given cell. The FSS transmitter 140 transmits an uplink signal to an FSS receiver 150 which in this example is a GSO satellite receiver. The LMDS hub 110-1 transmits and receives signals from the LMDS subscriber 115 and as noted above may utilize an LMDS repeater 160 to communicate with other subscribers in a corresponding repeater sub-cell. The desired FSS uplink and LMDS signal paths are shown as solid lines. The dashed lines indicate undesirable interfering signals. It can be seen from FIG. 2 that the FSS transmitter uplink signal represents an interfering signal to the LMDS subscriber 115, the LMDS hub 110-1 and the LMDS repeater 160. The interference power generated is directly proportional to the FSS earth station antenna sidelobe level in the direction of the LMDS subscriber 115. A single FSS transmitter can thus simultaneously interfere with many different LMDS subscriber receivers, as well as LMDS hub and repeater receivers. The problem is magnified when the FSS uplink experiences rain attenuation since proposed system designs implement power control to adaptively increase the transmitted power under heavy rain conditions. Similar interference problems arise between other types of terrestrial and space-based systems.
Prior art techniques for resolving these and other interference problems include full allocation of a given frequency band to either FSS or LMDS systems, or band segmentation in which both FSS and LMDS systems would receive less bandwidth than desired. For example, a band segmentation approach has been proposed for resolving the potential interference between FSS uplinks and LMDS subscriber receivers in the above-noted 28 GHz frequency band. However, these known techniques unduly restrict the use of a given frequency band and thereby prevent optimal delivery of the above-noted communication services to subscribers.
As is apparent from the above, a need exists for improved interference reduction which allows simultaneous operation of shared-frequency terrestrial and space-based communication systems.